Endochondral ossification involves a highly coordinated program of chondrocyte differentiation, proliferation, maturation and hypertrophy. The cellular activities of chondrocytes are organized in distinct zones in the growth plate, making this tissue uniquely suited for in vivo studies, as effects of mutations can be linked to specific cellular activities. The bone morphogenetic protein (BMP) and transforming growth factor beta (TGFb)/activin pathways are important regulators of these processes. The importance of extracellular antagonists as regulators of the duration, intensity and extent of BMP and TGFb/activin signaling has been defined. For example, mice lacking the BMP receptor antagonist Noggin exhibit cartilage overgrowth concurrent with excess BMP activity. However, very little is known about the roles of intracellular inhibitors of BMP and TGFb/activin pathways, such as the inhibitory Smad (I-Smad) proteins, Smad6 and Smad7. Gain and loss of function studies have revealed potential roles for I-Smads in chondrocytes in vitro, but no in vivo studies have been performed. The long-term goal of this project is to understand how intracellular regulation of BMP and TGFb/activin signaling by I-Smads controls chondrogenesis as a prerequisite to more efficient strategies utilizing BMPs to achieve cartilage maintenance and repair. To achieve this goal, it is important to determine whether I-Smads are critical for chondrogenesis and to identify specific aspects of chondrogenesis, as well as mechanisms, impacted by I-Smads. Hence, the specific hypotheses to be addressed in this proposal are (1) I-Smads are essential for normal endochondral bone formation, (2) loss of I-Smads impacts the balance of signaling cross-talk between TGFb/activin and BMP pathways in the growth plate that will affect the progression of endochondral ossification, and (3) TGFb signaling interact via Smad7 with key survival/stress pathways in the growth plate. PUBLIC HEALTH RELEVANCE: Arthritis is the nation's most common cause of disability [1], and current strategies for cartilage repair and maintenance include the use of recombinant human BMPs (rhBMPs) at supraphysiological doses. Inhibition of BMP signaling by inhibitory Smads (I-Smads) may account for the suboptimal effectiveness of rhBMPs on cartilage repair in the clinic, since I-Smads are induced directly by BMP signaling. Our proposed studies will provide a better understanding about the importance of the balance between signaling molecules that promote or inhibit endochondral bone formation, and may reveal the need for manipulating the balance between these proteins to promote more efficient BMP-induced cartilage regeneration than is currently possible. Reference 1: American Academy of Orthopaedic Surgeons (Eds.). (2008). Burden of Musculoskeletal Diseases in the United States: Prevalence, Societal and Economic Cost. Rosemont, IL: American Academy of Orthopaedic Surgeons.